1. Field of the Invention
The present invention relates to a surface treatment method and a surface treatment apparatus.
More particularly, the present invention relates to a method of forming a thin layer of polycrystalline silicon on a substrate, and to a method of forming a channel of a metal insulation semiconductor (MIS) transistor using the method of forming a thin layer of polycrystalline silicon. Also, more particularly, the present invention relates to a surface treatment apparatus such as a laser light emitting apparatus adaptive to perform, for example, a laser light annealing treatment, where the area of irradiation by a single shot of laser light is relatively large and an energy density of irradiated laser light is high, to thereby form, for example, a thin layer of polycrystalline silicon from an amorphous silicon layer.
2. Description of Related Art
First, the formation of a thin layer (film) of polycrystalline silicon on a substrate of a transistor will be described.
Thin film transistors (hereinafter referred to as TFTs) using amorphous silicon or polycrystalline silicon thin films are used for the transistors for driving pixels in liquid crystal displays, the transistors of peripheral elements, load element type static RAMs (hereinafter referred to as SRAMs), etc.
Polycrystalline silicon, however, has a higher density of unbonded arms of silicon atoms compared with monocrystalline silicon, so these unbonded arms become the cause of generation of a leakage current at the time the transistors are switched off. As a result, they become a cause of a reduced operating speed at the time of switching the transistors on. Accordingly, to improve the characteristics of a TFT, it is demanded to form a polycrystalline silicon thin film with few crystal defects and superior uniformity. As a method for formation of such a polycrystalline silicon thin film, the chemical vapor deposition method and the solid-state deposition method have been proposed. As the means for reducing the unbonded arms causing leakage current etc., use is made of the hydrogenation technique of doping hydrogen in the polycrystalline silicon thin film so as to cause hydrogen to bond with the unbonded arms.
If the chemical vapor deposition method is used for growing large grain size crystals and forming a polycrystalline silicon thin film, however, the thickness becomes nonuniform. With the chemical vapor deposition method, it is difficult to obtain a polycrystalline silicon thin film with a uniform thickness. Accordingly, it becomes difficult to form transistors with uniform device characteristics using a polycrystalline silicon thin film.
Further, with the solid-state deposition method, it is possible to grow dendrites and form a crystal with a grain size of at least 1 .mu.m, but the dendrites include small defects such as dislocation and twins which act as traps to obstruct improvement of characteristics and destabilize the operation.
To reduce the grain boundary trap density due to the unbonded arms, there has been proposed the method of performing annealing treatment using excimer laser light. Excimer laser light has the advantage of a large coefficient of absorption in silicon due to the UV light and can heat just the area near the surface of the silicon. Accordingly, there is no effect on the underlying layer, i.e., glass substrate, bottom layer LSI bonding portion, etc.
As methods for excimer laser annealing, there is known, first, the method of direct annealing of the amorphous silicon film and, second, the method of annealing by excimer laser light by an energy density at which the film as a whole will not melt.
The former method of direct annealing of the amorphous silicon is simpler as a process compared with the latter method and is advantageous for the future mass production of LSIs. Further, if a large area can be annealed by a single irradiation of the excimer laser light, this would be further advantageous for mass production.
When use is made of such excimer laser for directly annealing an amorphous silicon film, however, it is difficult to obtain an excimer laser beam able to cover a large area in a single shot and having in-plane uniformity sufficient for obtaining a polycrystalline silicon thin film having a low grain boundary trap density and a good crystallinity. To make up for this, an excimer laser having a large output energy able to anneal a large area with a single shot has been under development in recent years. Further, to improve the effect of the excimer laser annealing, it has been considered to heat the substrate to several hundred degrees for direct annealing of the amorphous silicon, but the process conditions for obtaining a polycrystalline silicon thin film having a low grain boundary trap density and good crystallinity have not been specified.
Further, in the above method of direct annealing of amorphous silicon, the grain size of the polycrystalline silicon was less than an average 50 nm.
Next, a surface treatment apparatus will be described.
When annealing a shallow region of a semiconductor substrate, use is made of a surface treatment apparatus which is provided with a laser emitting laser light of a short wavelength, for example, xenon chloride excimer laser light of a wavelength of 308 nm. The energy of the laser light emitted from a conventional excimer laser apparatus, however, is low, so it is not possible to anneal a wide area of about 30 mm.times.30 mm with a single shot of laser light.
Accordingly, to perform laser annealing of a wide area, the practice has been to form the laser light into a beam of for example about 0.6 cm.times.0.6 cm to raise the energy density to about 0.9 J/cm.sup.2 and to scan the annealing region with continuously emitted pulses of laser light.
If laser annealing is performed by this method, however, the time for.the annealing treatment becomes extremely long.
As shown in the explanatory view of the state of irradiation of laser light in FIG. 1, since the laser light 121 is irradiated in several pulses, the portion 141 connecting the region 131a irradiated by one shot of laser light 121a (121) and the region 131b irradiated by the next shot of laser light 121b (121) becomes discontinuous in terms of the amount of energy irradiated. Further, as shown by the graph of the density of irradiated energy and the irradiation position of FIG. 1, the laser emission output fluctuates among the shots 121c, 121d, and 121e, so the uniformity of the annealing treatment becomes poor in some parts. Therefore, it was difficult to apply this technique to manufacturing processes of polycrystalline silicon thin films.